The present invention relates to a method for fabricating a semiconductor device, and more particularly, to a method for forming a drain contact plug of a NAND-type flash memory device.
Recently, the demand for flash memory devices has increased. Flash memory devices can be electrically programmed and erased. Programming refers to writing data to a memory cell. Erasing refers to removing data written to a memory cell. Flash memory devices do not require a refresh function which re-writes data at regular intervals. Research on large scale integration technology has been actively conducted to develop a large-scaled semiconductor device which can store a large amount of data.
A NAND-type flash memory device is developed for the large scale integration of a memory device. In a NAND-type flash memory device, a plurality of memory cells is coupled in series to configure a string, i.e., to form a structure where adjacent cells share a drain and a source. The NAND-type flash memory device is a memory device which sequentially reads data, unlike a NOR-type flash memory device. The programming and the erasing of the NAND-type flash memory device is performed by implanting electrons into a floating gate and emitting the electrons using a Fowler-Nordheim (F-N) tunneling method to control a threshold voltage (Vt).
The NAND-type flash memory device transmits an externally supplied driving voltage, e.g., a bias voltage, to a lower semiconductor structure through a metal line. For instance, the lower semiconductor structure includes a source region and a drain region which are junction regions. A contact plug is generally required to electrically couple the metal line and the source and drain regions.
The contact plug of the NAND-type flash memory device includes a source contact plug (SRCT) and a drain contact plug (DRCT). The source contact plug couples the source region formed in an active region and an upper metal line, e.g., a source line. The drain contact plug couples the drain region and an upper metal line, e.g., a bit line.
Currently, a gate electrode is formed using a self-aligned shallow trench isolation (SA-STI) process in a NAND-type flash memory device under 70 nm to embody a large scale of integration and a micronized pattern.
However, in a gate structure formed using the SA-STI process, a seam in the shape of a keyhole may be generated in a polysilicon layer when forming the polysilicon layer for use as a drain contact plug in a contact hole formed in an insulation layer between gates.
FIGS. 1A to 1D illustrate cross-sectional views of a conventional method for fabricating a drain contact plug.
Referring to FIG. 1A, an insulation layer is formed to have a large thickness over a semi-finished substrate 10 including a source contact plug. A hard mask pattern 15 including a nitride-based material is formed over the insulation layer. The insulation layer is etched using the hard mask pattern 15 as a mask until the substrate 10 is exposed. Thus, a drain contact hole 17 is formed. Reference numeral 13 refers to a patterned insulation layer.
However, a bowing event (refer to ‘A’) may occur in the patterned insulation layer 13. In other words, portions of the patterned insulation layer 13 may be bent during the etch process when forming the drain contact hole 17 because the insulation layer has a large thickness to be etched. A critical dimension (CD) of the drain contact hole 17 in a portion where the bowing event occurs is represented with reference denotation CD1.
Referring to FIG. 1B, the hard mask pattern 15 is removed using an etch process. The bowing increases when the hard mask pattern 15 is removed. Therefore, the CD of the drain contact hole 17 becomes ‘CD2’. The hard mask pattern 15 is removed in advance because there may not be a slurry which can simultaneously polish the hard mask pattern 15, the patterned insulation layer 13, and a subsequent polysilicon layer (represented with reference numeral 19 in FIG. 1C) for forming a drain contact plug.
Referring to FIG. 1C, the polysilicon layer 19 for forming the drain contact plug is formed over the patterned insulation layer 13 and fills in the drain contact hole 17 (FIG. 1B). However, the drain contact hole 17 may not be completely filled when the polysilicon layer 19 is formed. A seam in the shape of a keyhole (refer to ‘B’) may be generated. This result is obtained because the polysilicon layer 19 having a sufficient step coverage characteristic is formed along the surface profile of the drain contact hole 17. The seam is generated at a certain depth (D1) from an upper surface of the patterned insulation layer 13.
Referring to FIG. 1D, a chemical mechanical polishing (CMP) process is performed to polish the polysilicon layer 19. Thus, a drain contact plug 19A is formed. A portion of the patterned insulation layer 13 is removed while polishing the polysilicon layer 19 and an upper portion of the seam is exposed (refer to ‘C’). Reference numeral 13A refers to a polished insulation layer 13A. The portion of the patterned insulation layer 13 is removed together with the polysilicon layer 19 because a slurry used for the polishing process has a polish selectivity ratio of the polysilicon layer 19 to the hard mask pattern 15 to the patterned insulation layer 13 in a range of approximately 1 to 2:0.3 to 0.4:1. There is a small difference between the polishing rate of the polysilicon 19 and the patterned insulation layer 13.
FIG. 2 illustrates a micrographic view showing the seam (refer to B) generated in the drain contact plug 19A in accordance with a conventional drain contact plug formation method. The seam generated in the shape of a keyhole in the drain contact hole 19A is exposed. Thus, a metal line M is formed in an undesirable manner over an upper portion of the drain contact plug 19A.
However, several limitations may occur during a subsequent metal line M formation process for transmitting a signal to the drain contact plug 19A when the seam is exposed. Thus, device characteristics deteriorate. For example, during a cleaning process performed before forming the metal line M, cleaning chemicals may not be sufficiently dried, the drain contact plug 19A may be damaged, a barrier metal layer may be undesirably formed, and unnecessary oxide-based materials including titanium oxide (TiO) may be generated. As a result, the likelihood of an erase failure may increase.